NHS Choices website analysed a scientific study which claimed that the levels of two stress hormones – cortisol and an enzyme produced in response to adrenaline levels (alpha amylase) – affected the likelihood of getting pregnant.
The researchers did find that women with higher salivary alpha-amylase levels were less likely to fall pregnant, compared with women with lower levels, but this was only of borderline statistical significance, and they concluded that the study failed to prove the effects of stress on natural conception and fertility.
I once asked a PhD student who was doing research into stress and infertility if she believed stress affected a woman's chances of conceiving. She said, "It’s well known that stress does not affect IVF treatment outcome, but it can affect natural conception." When I asked her why this was, she said it was because when a couple is very stressed in their lives, perhaps due to financial difficulties, serious problems at work or difficulties in their personal and family lives it has considerable impact on their relationship and quality of life. They may become emotionally and physically exhausted, they may argue a lot and fall out more, and generally be more unhappy. The direct consequence of this is that couples don't have sex as often or as much as they need to if they are trying to conceive. So, in this way, stress can affect their chances of pregnancy.
I realised then why everyone says “Just relax! You're too stressed" and that when a couple goes on holiday and they get on better and they have the odd glass of wine, they’re more likely to have sex and therefore more likely to get pregnant.